The present invention relates to rotary cam, expanding internal shoe drum brakes, and in particular, to an S-cam assembly having improved reserve lift capability and improved rollover protection.
Although the present invention is not strictly limited to rotary cam brakes of the S-cam type, but could be used with various other configurations of rotary cam brakes, the invention is especially advantageous when used with S-cam brakes, and will be described in connection therewith. S-cam expanding internal shoe drum brakes are well known in the prior art, as may be seen by reference to U.S. Pat. No. 4,905,800, assigned to the assignee of the present invention and incorporated herein by reference.
Although the conventional, prior art S-cam drum brakes have been commercially successful, those skilled in the art have continually attempted to increase lining wear between relinings. In general, increasing lining wear between relinings requires the use of thicker linings, which is generally satisfactory, but thicker linings require greater cam rise by the S-cam. Those skilled in the art have also attempted to improve the indication to the vehicle operator of the necessity of relining, and decrease the likelihood of cam flip (or rollover). It is believed that those skilled in the art are sufficiently familiar with cam flip, or rollover, and the causes thereof, such that no detailed explanation of the phenomenon is required. It is sufficient to note that, in the event of cam flip, or rollover, the cam would have rotated beyond its intended maximum rotary position, which would permit the brake shoes to move radially inward, out of engagement with the drum, such that no further braking action could occur.
In the above-incorporated U.S. Pat. No. 4,905,800, the working surfaces of the S-cam are provided at their ends with an extremely high rise/low mechanical advantage portion. After a substantial amount of lining wear, in order to move the brake shoes into engagement with the drum, sufficient rotation of the S-cam is necessary that the referenced high-rise, low mechanical advantage portions engage the cam followers, through which radially outward movement is imparted to the brake shoes, urging the brake shoes into engagement with the drums.
Although the improved S-cam of the referenced patent provides the vehicle operator with an indication (greater force required to rotate the S-cam), that the linings are worn, any such increase in "reserve lift" increases the overall diameter (minimum installation diameter) of the S-cam assembly. It is recognized by those skilled in the art that, in a typical installation, the amount of space between the brake shoe return spring and the inside diameter of the brake drum limits the ability to increase the reserve lift, utilizing any of the known prior art structures.